Wednesday, December 01, 2004

Output debate in Cornwall

A story from Devon reports on a debate sponsored by the Institute of Civil Engineers at the University of Plymouth. I note one item:
Mr Edwards [developer of the U.K.'s first wind facility] defended the efficiency of windfarms, saying: "At Delabole they generate for over 70 per cent of the time, and not 30 per cent as is often claimed."
This is a typical misleading response.

The 30% figure is the total annual output as a percentage of the facility's rated capacity. In the case of Delabole, ten 400-kilowatt turbines have the capacity of producing 10 × 400 KW × 24 hours × 365 days, i.e., 35 million kilowatt-hours/year. According to the utility that buys the power, they in fact produce 10 million, owing primarily to the variability of the wind. That is 28.5% of their capacity, which the developer generously rounds up to 30%.

That does not mean that the turbines are producing power only 30% of the time, because their output is proportional to wind speed. Typically they begin generating a trickle of electricity when the wind speed is around 8-10 mph, slowly building up to their maximum only when the wind speed is around 25-30 mph. If the wind gusts towards 60 mph, they shut down to prevent damage. The figure of 70% is the amount of time that the wind is within the range of, say, 10-60 mph and the turbines are responding.

So while his critics might cite the 30% output to suggest that the turbines are idle 70% of the time (or he skews their charge that way), Mr. Edwards cites the 70% activity as implying 70% output. In fact, because 30% (or, more accurately, less -- the average throughout the U.K. is 24%) is the average output, about two thirds of the time the turbines are producing less than that.

Edwards also mentions how "popular" his facility is (accepted might be more accurate). He neglects to note how small the ten 1991-vintage turbines are: less than half the height (and they don't need flashing lights) with blades sweeping an area just one fourth that of the typical models proposed on land today.